Generally, polymerization of an α-olefin in the presence of a metallocene catalyst employs methylaluminoxane or a boron compound as a promoter.
When a boron compound which is difficult to dissolve in a hydrocarbon solvent is employed as a promoter, in order to continuously supply the promoter in a homogeneous state to a reactor for polymerization of an α-olefin, a homogeneous catalyst is prepared before polymerization by bringing the boron compound into contact with a transition metal compound in a hydrocarbon solvent in the presence or absence of an organoaluminum compound (see, for example, Patent Document 1 or 2).
Such a technique facilitates supply of a catalyst to a polymerization reaction system. However, a catalyst used in such a reaction system is required to exhibit further improved polymerization activity.
Meanwhile, there has been proposed a method for preparing a catalyst exhibiting improved activity, by bringing a transition metal compound, a boron compound, and an α-olefin into contact with one another in a hydrocarbon solvent (see Patent Document 3).
However, the catalyst prepared through this technique is also required to exhibit further improved polymerization activity.